In a sense, mappers are already being told to do just that, by the statements by some that cycleway=track is deprecated. But I digress, that's just politics. :)

The technical difficulty is that up to now, neither of the two most-used rendering engines (mapnik and osmarender) could draw a line parallel to a way. As I said, work is being done on the mapnik render software right now, to incorporate just such a functionality. It still needs to be fleshed out and tested, but it opens up interesting possibilities in the future.

This is possibly the same problem: ​here is Canaan Lane, for the sections that are oneway there is a counterflow cycle lane, marked with cycleway=opposite_lane. This doesn't get rendered with Mapnik or Osmarender.

Now that Carto CSS is used to draw the default Mapnik style has the situation changed?
Carto supports any number of lines with or without offset to be drawn along the way. AFAIK it should be doable now, shouldn't it?